This invention relates generally to mechanical shift, pneumatic assist valves and more particularly to a mechanical shift pneumatic assist valve for diaphragm pumps which use a separate pilot valve to provide a positive signal (either on or off to the major air distribution valve).
Disclosed is an improvement of the device described in U.S. Pat. No. 4,854,832 assigned to The Aro Corporation. The prior art device significantly reduced the possibility of motor stall by providing a positive signal (either on or off) to the major air distribution valve. This was accomplished by adding a separate valve (pilot) which was not connected to the diaphragm rod. Actuation of the valve was accomplished by mechanically pushing the valve to the trip point with the diaphragm washer attached to the diaphragm connecting rod causing the major valve to shift. As pressure built up in the diaphragm air chamber it also acts on the end of the pilot rod (area) and forced it to end of its stroke. Air pressure holds it in this position until the diaphragm washer pushes it in the opposite direction. As long as the pilot rod was in either extreme position, a signal is always present to the major valve.
Other designs which incorporate the `pilot` on the diaphragm connecting rod, shut the signal off to the major valve after the diaphragm changes direction.
Occasionally an air pressure spike occurs in the diaphragm air chamber which is being exhausted. The spike occurs when there is an unusually rapid reversal of the diaphragms due to malfunctioning check valves or large volume of air trapped in one or both air caps or a restriction in the exhaust. If this pressure spike exceeds the pressure of the incoming air of the chamber being pressurized to pneumatically assist the trip rod, the spike can cause the trip rod to back up. Depending on the pump speed, operating pressure and severity of any one of the above conditions, the pump may begin to rapidly short stroke because the trip rod is oscillating back and forth around the trip point and out of sync with the diaphragm rod. Occasionally this condition results in a motor stall.
The foregoing illustrates limitations known to exist in present devices and methods. Thus, it is apparent that it would be advantageous to provide an alternative directed to overcoming one or more of the limitations set forth above. Accordingly, a suitable alternative is provided including features more fully disclosed hereinafter.